Today floodplain forests cover about 50,000 ha of the totaal surface area of Hungary. Floodplain forests and tree plantations are almost continuous along the Tisza. These are mainly soft wood forests (White and Black poplar), and on some smaller spots hard wood forests (oak-elm-ash forests). The so called 'kubik forests' (trees accompanying the clay pits, pollard willows) are made of Salix fragilis and S. alba. This habitat is very important, because the great number of old willow trees with hollows provide an important breeding place for many species. Unfortunately large areas are covered by tree plantations, with the different varieties of hybrid poplar. Beyond occupying the anyway narrow floodplain, these plantations cause the hybridisation of Black poplar and White poplar with the artificial hybrids, causing the genetic degradation of these two native species. Two nonnative tree species (Acer negundo and Fraxinus pennsylvanica) and a shrub (Amorpha fruticosa) cause additional problems with their aggressive expansion.